Insulated gate power semiconductor devices, such as metal oxide semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs), are widely used as semiconductor switching elements. In a typical example thereof, a switching element can be on by applying a voltage equal to or higher than a threshold voltage to a gate electrode to form a channel. Especially in a trench gate power semiconductor device, a semiconductor layer has a trench, and a base region on a side face of the trench is used as the channel. This improves a channel width density, and thus enables reduction in cell pitch to thereby improve device performance.
As a semiconductor material for semiconductor switching elements, a wide bandgap semiconductor has attracted attention in recent years to yield high-voltage, low-loss semiconductor switching elements. The wide bandgap semiconductor shows promise for the application especially in the technical field of using a high voltage of approximately 1 kV or higher. Examples of the wide bandgap semiconductor include, in addition to SiC, a gallium nitride (GaN) material or diamond. In a trench gate silicon carbide semiconductor device including the wide bandgap semiconductor, a gate insulating film, such as a silicon oxide film, is likely to have a breakdown field strength approximately equal to an avalanche field strength in a pn junction between a base region and a drift layer. Consideration for both the strengths is required to increase the breakdown voltage.
Some vertical power semiconductor devices, which are one type of power semiconductor device, include a plurality of unit cells partitioned by gate electrodes and connected in parallel to each other. The semiconductor device can be classified by arrangement pattern of the unit cells. A cell-type semiconductor device and a stripe-type semiconductor device are typical examples. In the cell-type semiconductor device, one unit cell includes a source region formed in a square pattern and a gate trench surrounding the source region. In the stripe-type semiconductor device, source regions are formed in an elongated striped pattern, and a gate trench is located between any two source regions. A plurality of unit cells constitute an element region functioning as a semiconductor element, and a termination region is located to surround the element region.
A peripheral portion of the element region adjacent to the termination region and a portion of the element region inside the peripheral portion have different field states due to their different surrounding configurations. The field strength can thus become particularly high in the peripheral portion at application of reverse bias. The breakdown voltage of the semiconductor device is determined by the minimum breakdown voltage of individual cells, and thus cells in the peripheral portion preferably have an equal breakdown voltage to cells in the inside portion. The configuration to increase the breakdown voltage of the cells in the peripheral portion has been studied. According to Japanese Patent Application Laid-Open Publication No. 2005-322949 (Patent Document 1), for example, a trench has been stretched from an element region to a termination region to prevent the occurrence of a high field in a peripheral portion of the element region. This prevents breakdown of a gate insulating film to thereby improve the breakdown voltage of the semiconductor device.
In addition to the above-mentioned technique, many techniques of providing a diffusion region having a conductivity type opposite the conductivity type of a drift layer at a deeper location than a trench to prevent breakdown of a gate insulating film of a SiC semiconductor device are disclosed. According to WO 98/35390 (Patent Document 2), for example, a protective region having a conductivity type opposite the conductivity type of a drain region is formed at the bottom of a gate trench. According to Japanese Patent Application Laid-Open Publication No. 2009-194065 (Patent Document 3), a p-type deep layer is formed, in a direction orthogonal to a gate trench, in an n−drift layer at a location lower than a p-type base region. According to Japanese Patent Application Laid-Open Publication No. 2012-178536 (Patent Document 4), a source trench is formed in a silicon carbide semiconductor to reach an n−drift layer, and a p-type source-breakdown-voltage-holding region is formed at the bottom of the source trench. Such a diffusion region relieves field crowding at the bottom of the trench of a gate electrode when a MOSFET is off. The breakdown voltage of a switching element is thus increased.
Important characteristics that the switching element is desired to have include, in addition to a high breakdown voltage, a low on-resistance. According to WO 98/35390 (Patent Document 2) as described above, a trench gate silicon carbide semiconductor device includes, between a p-type base region and an n-type drift layer, an n-type current diffusion layer having a higher impurity concentration than the drift layer. The current diffusion layer allows current having passed through a channel formed in the base region on a side face of the trench to diffusely flow through the current diffusion layer in a lateral direction. This can reduce the on-resistance.